The numbers ofAcanthamoebabinding to new hydrogel contact lenses of different polymer and water content were determined with two quantitative methods, a radiolabeled-cell method and a detaching-fluid method. Numbers of amoebae retained on nonionic lenses increased with increasing water content of the lenses. With both nonionic and ionic lenses numbers of associated amoebae decreased with successive rinsing steps. The retentions of amoebae on unworn hydrogel lenses, in contrast to the irreversible adhesion of bacteria, were tenuous and appeared to be effected mainly by surface tension, surface charge and water content.